1. Field of the Invention
The present invention relates to an optical pickup apparatus, and more particularly, to an optical pickup for detecting thickness variation of a recording medium, and/or for compensating for spherical aberration caused by the thickness variation of a recording medium.
2. Description of the Related Art
In general, information recording/reproduction density increases as a size of a light spot focused on a recording medium in an optical pickup apparatus becomes smaller. The shorter a wavelength (λ) of light used and the larger a numerical aperture (NA) of an objective lens, the smaller the size of a light spot, which is expressed by equation (1):size of light spot α λ/NA  (1)
To reduce the size of the light spot focused on the recording medium in order to obtain a higher recording density, there is a need to construct an optical pickup with a short wavelength light source, such as a blue semiconductor laser, and an objective lens having a larger NA. A format for increasing recording capacity up to 22.5 GB with a 0.85-NA objective lens, and for reducing the thickness of a recording medium to 0.1 mm is desired so as to prevent degradation of performance caused by tilting of the recording medium. Here, the thickness of the recording medium is defined as a distance from a light incident surface of the recording medium to an information recording surface.
As shown in equation (2) below, an spherical aberration W is proportional to a fourth power of the NA of the objective lens and to a deviation of the thickness of the recording medium. For this reason, if an objective lens with a high NA of about 0.85 is adopted, the recording medium must have a uniform-thickness with a deviation less than ±3 μm. However, it is very difficult to manufacture the recording medium within the above thickness deviation range.
                              W                      [            40            ]                          =                                                            n                2                            -              1                                      8              ⁢                              n                3                                              ⁢                                    (              NA              )                        4                    ⁢          Δ          ⁢                                          ⁢          d                                    (        2        )            
FIG. 1 is a graph showing a relation between thickness deviation of the recording medium and wavefront aberration (optical path difference (OPD)) caused by a thickness deviation when a 400-nm light source and an objective lens having an NA of 0.85 are used. As shown in FIG. 1, the wavefront aberration increases proportionally with the thickness deviation. Thus, when the objective lens having a high NA, for example, an NA of 0.85, is adopted, there is a need to correct for spherical aberration caused by the thickness deviation of the recording medium.
FIG. 2 shows a conventional optical pickup capable of detecting variation of the thickness of an optical disc 1, which is disclosed in Japanese Patent Laid-open Publication No. hei 12-57616. Referring to FIG. 2, the conventional optical pickup includes a light source 10 emitting a light beam, a polarization beam splitter 11 transmitting or reflecting the light beam from the light source 10 incident on the optical disc 1 according to the polarization of the light beam, and a quarter-wave plate 15 changing the polarization of an incident light beam. An objective lens 17 focuses the incident light beam to form a light spot on a recording surface 1a of the optical disc 1. A cylindrical astigmatism lens 21 has an astigmatism affecting the light beam passed back through the objective lens 17, the quarter-wave plate 15, and the polarization beam splitter 11 after being reflected from the recording surface 1a of the optical disc 1. A photodetector 25 receives the light beam from the astigmatism lens 21. The conventional optical pickup further includes a collimating lens 13 disposed between the polarization beam splitter 11 and the quarter-wave plate 15, collimating an incident diverging light beam from the light source 10 transmitted or reflected by the polarization beam splitter 11. A condensing lens 19 is disposed between the polarization beam splitter 11 and the astigmatism lens 21. The polarization beam splitter 11, the collimator lens 13, the quarter-wave plate 15, the objective lens 17, the condensing lens 19, and the cylindrical astigmatism lens 21 are coaxially arranged.
Because the conventional optical pickup has the astigmatism lens 21 which causes astigmatism to enable focus error signal detection, the intensity distribution of the light passed through the astigmatism lens 21 after being reflected on the recording surface 1a of the optical disc 1 varies according to the thickness t′ of the optical disc 1, as shown in FIGS. 3A through 3E. FIGS. 3A through 3E illustrate an intensity distribution of light passed through the astigmatism lens 21 towards the photodetector 25, when the optical disc 1 adopted has a thickness of 0.70 mm, 0.65 mm, 0.60 mm, 0.55 mm, and 0.50 mm, respectively, and the optical pickup of FIG. 2 is designed for a 0.6-mm thick optical disc.
Referring to FIG. 3C, when the optical disc 1 has a thickness of 0.60 mm, which is a level of reference with respect to the other thickness levels (hereinafter, referred to as the reference thickness), the intensity distribution of the reflected light beam entering the photodetector 25 is circular due to lack of spherical aberration, and is symmetrical around a center point. When the thickness of the optical disc 1 deviates from the reference thickness of 0.60 mm, spherical aberration occurs as a result of the thickness deviation, and the intensity distribution of the reflected light beam passed through the astigmatism lens 21 and received by the photodetector 25 is asymmetrical about the center point, as illustrated in FIGS. 3A, 3B, 3D, and 3E.
The photodetector 25 detects a variation in thickness of the optical disc 1 from a variation of intensity distribution of the received light. To this end, as shown in FIG. 4, the photodetector 25 of FIG. 2 includes first through fourth inner sections A1, B1, C1, and D1, and first through fourth outer sections A2, B2, C2, and D2 surrounding the first through fourth inner sections A1, B1, C1, and D1.
In a conventional optical pickup having the configuration described above, a thickness variation signal for the optical disc 1 is detected by subtracting a sum of detection signals a2 and c2 of the first and third outer sections A2 and C2 in one diagonal direction of the photodetector 25, and the detection signals b1 and d1 of the second and fourth inner sections B1 and D1, respectively, in the other diagonal direction, from a sum of detection signals a1 and c1 of the first and third inner sections A1 and C1, respectively, in the one diagonal direction, and detection signals b2 and d2 of the second and fourth outer sections B2 and D2, respectively, in the other diagonal direction. In other words, a thickness variation signal St′ for the optical disc 1 can be detected from the detection signals a1, b1, c1, and d1 of the first through fourth inner sections A1, B1, C1, and D1, respectively, of the photodetector 25, and the detection signals a2, b2, c2, and d2 of the first through fourth outer sections A2, B2, C2 and D2, respectively, by using the following equation:St′=(a1+c1+b2+d2)−(a2+c2+b1+d1)  (3)
However, this mechanism of detecting variation of the thickness of the optical disc can be applied to only optical pickups adopting the astigmatism lens. In other words, if an optical pickup does not include the astigmatism lens, a thickness variation of an optical disc used in the optical pickup cannot be detected.